1. Field of the Invention
This invention relates generally to heat exchangers, and more particularly to a heat exchanger for transferring heat between a fluid on the inside of a wall and a fluid at a different pressure on the outside of the wall, and a method of constructing such a heat exchanger.
2. Description of the Related Art
Heat exchangers transfer heat energy from one fluid to another. A common heat exchanger is an automobile radiator, in which heat is transferred from a warm water solution in the radiator to the cooler air. Heat is removed by passing the fluid, which can be a liquid or gas, through a thin-walled passage and directing air over the outside of the thin-walled passage. Gas molecules in the air impinge upon the walls of the passage, removing heat during contact.
In free piston Stirling cycle machines, there is a need to transfer heat from a gas on one side of a hermetically sealed housing to a fluid, such as environmental air, on the other. In free piston Stirling cycle cryocoolers in particular, a working gas, such as helium, within the housing is compressed, thereby raising its temperature. Heat is removed from the compression region of the housing as part of the process of absorbing heat in one region of the housing and rejecting it at another.
This heat pumping process requires the flow of heat energy through the housing wall. However, the most common housing wall material, stainless steel, is not a particularly good thermal conductor. A housing wall that is made thinner to transfer heat more rapidly cannot support the pressure within the housing.
Heat transfer in conventional Stirling cycle machines is assisted by attaching thin, highly thermally conductive fins to the inside and outside of the housing to promote heat transfer. The internal fins have high surface area upon which the working gas within the machine impinges, transferring heat energy to the fins. This heat energy flows through the housing wall to the cooler fins on the outside of the housing. Fluid coolant, such as ambient air, passes over the outer fins, removing heat.
Fins are conventionally attached by one of two methods. In one method, fins are brazed or soldered to the interior and exterior surfaces of the housing. In the second method, the housing is separated into two sections by cutting along a plane intersecting the housing. A fin structure is interposed between the two housing sections and brazed or soldered into place.
Two disadvantages to soldering or brazing fins to the housing are the high cost and the tendency brazing and soldering have to modify the metallurgical properties of both the housing and the fins. Disadvantages of interposing a fin structure include the high costs and metallurgical effects, and the possibility of leaks due to poor soldering or brazing.
Therefore, the need exists for an effective heat exchanger, and a method for forming the same, on a Stirling cycle machine in particular, and opposing sides of walls in general.
The invention is a heat exchanger for transferring heat energy from one side of a housing wall to the opposite side. The invention also contemplates a method of constructing the heat exchanger. In the preferred embodiment, the housing wall is the housing of a free piston Stirling cycle machine, such as a cryocooler.
The apparatus includes an inner ring that seats against the inner surface of the housing. An outer ring seats against an outer surface of the housing. The rings are positioned coaxially and aligned longitudinally on opposite sides of the housing wall, forming a thermal energy conduction path from ring to ring. The rings also support the housing wall under the stress created by the pressure within the housing.
Heat transfer means, preferably thin, highly thermally conductive fins, are mounted to the opposing sides of the rings. The inner fins promote conduction of heat from the working gas within the housing to the inner ring. The heat is conducted through the housing sidewall to the outer ring. The heat is then conducted to the outer fins and then removed by gas circulating through gaps between the outer fins. This gas is environmental air in the embodiment contemplated, but could alternatively be a fluid coolant.
A method of forming the apparatus comprises seating the inner ring against the interior surface of the housing and then displacing it longitudinally to a predetermined longitudinal position. The outer ring is seated against the exterior surface of the housing and displaced longitudinally to the predetermined longitudinal position, preferably aligned with the inner ring on the opposite side of the sidewall. The relative temperatures of the rings can also be changed if desired.
The heat exchanger constructed has an interference fit between the abutting surfaces of the rings and the housing sidewall, thereby preventing relative movement of the rings and the housing sidewall. Furthermore, the high-contact area between the rings and the housing provides an excellent path for thermal energy conduction. There is no weakening of the metallurgical properties of the housing due to soldering or brazing, and in fact the heat exchanger strengthens the housing. There is no need to re-seal the housing sidewall due to interposition of a structure.